hterm: Recognize italics CSI escape codes.

\E[3m is now recognized as italics.
\E[23m is now recognized as unitalics.

These escape codes are not supported by xterm, but they are supported by
other terminals such as gnome-terminal and rxvt, and partially supported
by screen and tmux. They are listed as ANSI escape codes on wikipedia:
http://en.wikipedia.org/wiki/ANSI_escape_code.

Change-Id: I5dcadd3e6ba23b0f15835c0e3dbcb4e2c91ed89c
Reviewed-on: https://chromium-review.googlesource.com/200840
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>